Molecular sieving carbon and activated carbon are manufactured from carbon compounds obtained by the carbonization of carbon compounds such as coke, coal, wood charcoal, and coconut shell char; and all types of resin such as phenol resin, furan resin, and vinylidene chloride copolymer. In the present invention material obtained by the carbonization of carbon compounds is called carbonized charcoal.
Molecular sieving carbons (abbreviated as MSC, and also called carbon molecular sieves, abbreviated as CMS) are known as adsorbents for separating air into nitrogen and oxygen.
Molecular sieving carbons are adsorption-rate-dependent-separation adsorbents which use the fact that oxygen which has a small molecular diameter has a faster rate of adsorption than nitrogen.
Molecular sieving carbons are obtained by means of adjusting the size of the diameter of the pores in carbonaceous material (for example, activated carbon) by means of various methods.
Examples of the manufacturing methods for molecular sieving carbons which have been proposed up till now are described in Japanese Patent Application, Second Publication, No. Sho 52-18675; Japanese Patent Application, First Publication, No. Sho 59-64514; Japanese Patent Application, Second Publication, No. Sho 61-8004; Japanese Patent Application, First Publication, No. Sho 62-176908; Japanese Patent Application, First Publication, No. Sho 60-171212; the specification of U.S. Pat. No. 5,098,880; Japanese Patent Application, First Publication, No. Sho 62-176908; Japanese Patent Application, First Publication, No. Sho 62-132543; Japanese Patent Application, First Publication, No. Sho 62-108722; Japanese Patent Application, Second Publication, No. Sho 49-18555; Japanese Patent Application, Second Publication, No. Sho 61-8004; Japanese Patent Application, First Publication, No. Hei 4-310209, and so on.
Japanese Patent Application, Second Publication, No. Sho 52-18675 discloses a method in which hydrocarbons which discharge carbon by means of thermal decomposition are added to coke which contains a volatile component of up to 5%, and treated at 600.about.900.degree. C. Japanese Patent Application, First Publication, No. Sho 60-171212 discloses a method in which the size of the pore diameter of activated carbon is adjusted by means of impregnating a commercially available carbon adsorbent with a thermally decomposable hydrocarbon. The specification of U.S. Pat. No. 5,098,880 discloses a method for adjusting the size of the pore diameter, wherein activated carbon in which two types of volatile hydrocarbon which have been divided into two portions are brought into contact with commercially available activated carbon starting material while heating.
As a conventional pressure swing adsorption method for separating air using a molecular sieving carbon, the specification of U.S. Pat. No. 2,944,627; Japanese Patent Application, Second Publication, No. Sho 53-44160; Japanese Patent Application, Second Publication, No. Sho 54-8200; and Japanese Patent Application, First Publication, No. Sho 59-182215 are known.
As methods for separating methane, carbon dioxide, and the like from a gas mixture, there are solution absorption methods, liquefied separation methods, and the like; however, pressure swing adsorption methods, called PSA (Pressure Swing Adsorption) methods, are more commonly used. Zeolite, activated carbon, and the like are used as adsorbents in pressure swing adsorption methods.
Japanese Patent Application, First Publication, No. Hei 3-98641 discloses that activated carbon which is superior in the adsorption of carbon dioxide can be obtained by means of treating carbonaceous material or activated carbon at approximately 600.degree. C. or less in an activating atmosphere which contains oxygen and under conditions in which combustion does not take place.
Japanese Patent Application, First Publication, No. Hei 4-200742 states that the adsorption at low temperature of carbon dioxide by activated carbon which contains amine and water followed by discharge at high temperature is suitable.
Japanese Patent Application, Second Publication, No. Sho 52-47758 discloses that a carbonaceous material which adsorbs carbon dioxide but does not adsorb methane can be obtained by crushing carbonized saran waste, mixing it with a sintering agent like coal tar, then, mixing it with a granulating agent such as Avicel, granulating it, and carbonizing it at 400.about.900.degree. C.
Japanese Patent Application, First Publication, No. Hei 6-100309 discloses that the difference in the equilibrium adsorption for methane and for carbon dioxide is large for a molecular sieving carbon obtained by putting a carbon substrate having an average micropore diameter of 5.5-12 .ANG. into a treatment oven heated to 650.about.850.degree. C., and conducting vapor deposition of thermally decomposed carbon on the micropores by supplying an inert gas which contains aromatic hydrocarbon and/or alicyclic hydrocarbon to the treatment oven.
As an example of the separation of carbon dioxide from a gas mixture by pressure swing adsorption methods using activated carbon, there are Japanese Patent Application, First Publication, No. Sho 60-241931, and Japanese Patent Application, First Publication, No. Hei 3-98641.
The following Examples give data for adsorption isotherms of carbon dioxide by activated carbon: Kawazoe et al., Seisankenkyu, 25, 11, page 513, 1973 [8.5 g/100 g=43 mlSTP/g (20.degree. C., 1 atm)]; Yano et al., Kagaku Kogaku, 25, 9, page 654, 1961, [30 ccSTP/g (30.degree. C., 1 atm)]; and Kagaku Kogaku Binran, page 589, 1992, [40 cm.sup.3 NTP/g (37.7.degree. C., 1 atm)].
In addition, the following examples give data for methane adsorption by activated carbon: Nitta et al., J. Chem. Eng. Jpn, Vol. 25, No. 2, page 176, 1992 [1 mol/kg=22.4 mlSTP/g (25.degree. C., 1 atm)]; Kimberly et al., Chem. Eng. Science, Vol. 47, No. 7, page 1569, [0.7.about.1.1 mmol/g=15.7.about.24.6 mlSTP/g (25.degree. C., 1 atm)]; and Kagaku Kogaku Binran, page 589, 1992, [21 cm.sup.3 NTP/g (37.7.degree. C., 1 atm)].
For molecular sieving carbon manufactured by conventional methods, the adsorption capacity for oxygen and the coefficient of separation of oxygen and nitrogen were insufficient. For this reason, air separation by means of pressure swing adsorption required large amounts of molecular sieving carbon for each production unit of nitrogen gas. In addition, since the nitrogen yield was insufficient, the separation energy for each unit of nitrogen gas was high.
The separation of carbon dioxide, methane, and the like has become practicable by means of pressure swing adsorption methods which use activated carbon. However, its adsorption capacity is not able to sufficiently satisfy, and provision of activated carbon which is superior in adsorption efficiency of carbon dioxide, methane, and the like is strongly desired.